R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
1
EFFECT OF BULK DENSITY ON HYDRAULIC PROPERTIES
OF HOMOGENIZED AND STRUCTURED SOILS
Dorota Dec1 , José Dörner1, Orsolya Becker-Fazekas2 and Rainer Horn3
1
Instituto de Ingeniería Agraria y Suelos, Universidad Austral de Chile, Casilla 567,
Valdivia, Chile.
2
Institute for Soil Science and Land Evaluation, University of Hohenheim, Germany
3
Institute for Plant Nutrition and Soil Science, Christian Albrechts
University of Kiel, Germany.
Corresponding autor: [email protected]
Efecto de la densidad aparente sobre las propiedades hidráulicas de los suelos
homogenizados y estructurados
Keywords: Water retention curve, soil shrinkage, soil structure
ABSTRACT
In order to determine the effect of bulk density and soil structure on their hydraulic
behaviour, undisturbed samples were collected and disturbed samples were prepared from
a Luvisol. The experimental field, located in Harste/Goettingen, Germany, was cultivated
with sugar beet (Beta vulgaris). The water retention curve (WRC) and saturated hydraulic
conductivity (Ks) were measured on undisturbed samples and homogenized samples
prepared at different bulk densities (1.2 -1.4 and 1.6 Mg m-3). To determine the effect of
soil shrinkage on the unsaturated hydraulic conductivity (k/ψ), the vertical deformation
of the repacked samples was measured and relative water content differences (dθ)were
determined. The hydraulic properties of soils with identical texture depend on bulk density
(dB) and structure. The increasing bulk density not only induces changes in the pore-size
distribution but also affects the ability of soil to shrink and to conduct water under
unsaturated conditions. Greater shrinkage was observed for samples with lower dB as a
consequence of reduction of coarse pores. The water content differences increase with
decreasing bulk density, inducing an increasing error in the estimation of k/ψ.
2
Physical properties in soils, Dec et al.
Palabras clave: Curva de retención de agua, contracción suelo, estructura suelo
RESUMEN
Con el objetivo de determinar el efecto de la densidad aparente y estructura del suelo sobre
su comportamiento hidráulico, muestras no disturbadas fueron recolectadas y muestras
disturbadas fueron preparadas a partir de un Luvisol. El campo experimental, ubicado en
Harste/Goettingen, Alemania, fue cultivado con remolacha (Beta vulgaris). La curva de
retención de agua (WRC) y la conductividad hidráulica saturada (Ks) fue determinada en
muestras no disturbadas de suelo y en muestras homogenizadas preparadas a distintas
densidades aparentes (1.2 -1.4 y 1.6 Mg m-3). Para determinar el efecto de la contracción
del suelo sobre la conductividad hidráulica no saturada (k/ψ), se midió la deformación
vertical de las muestras ensambladas y a partir de ello se determinó la diferencia en el
contenido volumétrico de agua.
Las propiedades hidráulicas de suelos con la misma textura dependen de la densidad
aparente y la estructura. El incremento en la densidad aparente no sólo induce cambios
en la distribución de la porosidad sino que también afecta la capacidad de contracción del
suelo y de conducir agua en fase no saturada. Una mayor contracción fue observada en
muestras con menor densidad aparente como consecuencia de la reducción de los poros
gruesos. Las diferencias en el contenido de agua aumentan con una disminución en la
densidad aparente, induciendo un mayor error en la estimación de (k/ψ).
INTRODUCTION
Soil hydraulic properties like the retention
capacity and hydraulic as well as gas
conductivity have agronomical and
ecological implications. Drainage,
evaporation and water-uptake by roots are
just few examples where the knowledge
about the rate of water flow through the soil
plays an important role (Plagge et al., 1990;
Kutilek and Nielsen, 1994).
The relative proportion of the three phases
(water, gas and solid) of the soil is influenced
by properties like texture, structure, biological
activity, weather and soil management
(Hillel, 1998). In these terms the porous
media can be characterized in their volume
and function which is of great relevance to
understand processes related to water, air
and heat transport in soils (Oschner et al.,
2001; Dörner and Horn, 2006).
Soil volumes are affected by mechanical
stresses (e.g. tillage-induced soil compaction,
Blackwell et al., 1986; Horn et al., 1991;
Horn et al., 1995; Ball et al., 1997; McNabb
et al, 2001) and internal forces (e.g. wetting
and drying cycles, Peng and Horn, 2007;
Bartoli et al., 2007). The magnitude of
volume changes are controlled by the
mechanical stability of the soil or by the
level of the actual in comparison with
previous internal stresses. The internal
stresses in soils containing more than 12%
of clay induce aggregate formation, which
results firstly in a prismatic structure with
a dominant vertical pore function
orientation. Repeated swelling and
shrinkage creates tensile and shear induced
crack formation in blocky and thereafter in
a subangular blocky structure (Horn and
Smucker, 2005).
The effects of soil compaction on soil
structure have been investigated by many
authors (Blackwell et al., 1986; Horn et al.,
1995; Ball et al., 1997; McNabb et al.,
2001). It is also well known that soils are
able to shrink and swell (Peng et al., 2006).
However, with respect to the calculation of
hydraulic properties, it is always assumed
that soils behave like a rigid body. Soil
R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
shrinkage can be characterized by the
shrinkage curve, which relates the volume
changes as a function of the water content
changes. The shrinkage curve generally
presents a typical sigmoidal shape; with
linear and curvilinear parts determining
successive phases of shrinkage. Bronswijk
(1990) defined these phases from the wet
to the dry side as: structural shrinkage,
normal shrinkage, residual shrinkage and
zero shrinkage. These zones differ between
each other from the relative proportion of
water losses and volume decrease being
both proportional in the normal shrinkage
but not in structural and residual shrinkage,
where the water losses exceeds volume
decrease.
Soil structure is changing continuously due
to external and internal forces affecting
consequently soil porosity and its functions.
In order to describe the hydraulic behaviour
of structured and disturbed soils with
different compaction level, the objectives
of this work were to determine: (1) the effect
3
of bulk density on the water retention curve,
(2) soil shrinkage and (3) its consequences
on the hydraulic conductivity.
MATERIALS AND METHODS
Soil material and management
The investigation was conducted on arable
soils in a slightly undulating area in Northern
Germany. The soil is described as Stagnic
Luvisol (FAO, 1998) derived from Loess
with two tillage treatments: Conservation
(Mulch-M) and Conventional (Plough-P).
Mulch defines the treatment when the soil
is loosened with a field cultivator to a depth
of 8-10 cm, while Plough means ploughing
and complete converting down to 30 cm
depth. Before 1992 the whole field was
uniformly ploughed to 30 cm depth. At the
sampling time the soil was cultivated with
sugar beet (Beta vulgaris). Some physical
properties of the studied soils are shown in
Table 1.
Table 1. Physical properties of the studied soils
Cuadro 1. Propiedades físicas de los suelos estudiados
Site
Sand
Silt
Clay
dB
-
TP
Corg
[%]
[%]
[%]
[Mg m ]
[%]
[Mg g-1]
Ph
3
80
17
1.2-1.4-1.6
55-47-39
1.4
Ps
3
80
17
1.53
42
1.4
Mh
3
80
17
1.2-1.4-1.6
55-47-39
1.4
Ms
3
80
17
1.47
44
1.4
Ph -homogenized soil samples prepared from Plough; Ps -structured soil samples taken
from Plough; Mh -homogenized soil samples prepared from Mulch; Ms-structured soil
samples taken from Mulch; dB -bulk density [Mg m-3]; TP -total porosity [%]; Corg-organic
carbon content [Mg g-1]
4
Physical properties in soils, Dec et al.
Collection of undisturbed soil samples
The soil sampling took place in November
2003, directly after sugar beet harvesting.
Undisturbed soil samples were taken at a
depth of 15-19 cm in metal cylinders with
6 replications. Afterwards, the soil samples
were put into plastic bags and boxes to avoid
evaporation and mechanical disturbances.
Preparation of repacked soil samples by
three bulk densities
In addition, from the sieved soil (<2 mm)
repacked soil samples with three different
bulk densities (dB) were prepared: 1.2, 1.4
and 1.6 Mg m-3 (ten replications for each
bulk density prepared with 20% gravimetric
water content). To ensure uniform bulk
densities, the soil was packed into cylinders
at carefully controlled densities by means
of a “Load frame” device.
Measurements
In order to determine the effect of bulk
density on hydraulic properties four samples
were used to measure the water retention
curve (WRC) and shrinkage (S), and six to
measure the saturated hydraulic conductivity
(Ks). First all samples were slowly saturated
by capillary rise from beneath. The
saturation took at least three days.
Determination of the water retention
curve and soil shrinkage
The water retention curve (WRC) was
determined at 8 different water suctions.
The saturated samples were drained at-10,
-20 and -30 hPa on sand tanks. The drainage
of the samples at water tensions of -60, 150, -300, -500 hPa occurred on ceramic
plates and of -15000 hPa in a pressure
chamber. When the samples attained
equilibrium with the applied vacuum, the
gravimetric water content was determined.
At the end, samples were dried in an oven
at 105 °C for 16 hours (Hartge and Horn,
1989) to determine and to control the bulk
density and volumetric water content. To
determine the total porosity, a particle
density of 2.63 Mg m-3 was used resulting
from the dominant quartz.
The shrinkage of the repacked samples was
measured during the determination of the
WRC. To estimate the vertical shrinkage
of the soil their actual height was measured
with a calliper at 6 defined points (1 on the
middle and 5 on the sides) as was proposed
by Peng and Horn (2005).
Determination of the saturated hydraulic
conductivity
The saturated hydraulic conductivity (Ks)
was measured with a permeameter under
instationary conditions. The water flow was
measured three times for each soil sample
and the arithmetical mean was calculated.
The Ks values were log-transformed because
of their non-normal distribution (Hartge and
Horn, 1989).
Correction of the water retention curve
after shrinkage
In order to consider the shrinkage effect on
the water retention curve, the vertical
shrinkage was used to correct the volumetric
water content at each matric potential.
Additionally, the structural changes caused
by shrinkage were expressed by differences
in water content (dθ) with and without
consideration of shrinkage, in relation to
the volumetric water content at saturation
as follows:
dθ =
θ rs − θ dh
⋅ 100
θs
(1)
where θrs means the volumetric water content
of rigid soil [vol%], θdh is the volumetric
water content corrected by shrinkage [vol%]
and θs is the volumetric water content at
saturation [vol%].
At the used water tensions (till -500 hPa)
horizontal cracks were not clearly observed
in the investigated soils. Additionally,
regarding the complicated and irregular
geometry of soil cracks, the quantitative
and feasible measurement of horizontal
shrinkage is more difficult as of the vertical
(Peng et al., 2006). Therefore, our
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
investigations were concentrated only on
the vertical changes. Such restriction can
be justified by the initial dominance of
vertical cracks during shrinkage, as it is
described in Hartge and Horn (1999).
In order to show the effect of the bulk
density and soil shrinkage on the water
movement under unsaturated conditions,
the hydraulic conductivity as function of
the water tension was estimated using the
equation (2) proposed by van Genuchten
(1980), with and without consideration of
the soil deformation as follows:
k /ψ = Ks ⋅
[1 − (α ⋅ h)
n −1
(
)
]
2
n −m
⋅ 1 + (α ⋅ h )
[1 + (α ⋅ h) ]
n m/2
(2)
where k/ψ means the unsaturated hydraulic
conductivity [cm s-1], h is the water pressure
[hPa], Ks is the saturated hydraulic
conductivity [cm s-1] and α, n, m are the van
Genuchten parameter of the WRC [-].
Statistical analysis
Averages for each soil horizon were
calculated. In order to proof the effect of
bulk density and structure formation on
pore-size distribution and Ks, statistical
analysis was performed by analysis of
variance (p≤0.05). The differences of means
were assessed by the Tukey-Test (p≤0.05).
RESULTS
Effect of the bulk density on the water
retention curve and soil shrinkage
The development of the WRC depends on
the soil density as well as on soil structure
(Fig. 1). As expected, the saturated water
content (θs) decreases with increasing bulk
density (Tab. 2). The development of the
WRC for the structured samples (Ps and
Ms) differs from that of the homogenized
material (Ph and Mh), even if they have a
nearly similar water content at saturation.
It is noticeable that the
Content [Vol%]
Water Content [Vol%]
Water Content [Vol%]
Water ContentWater
[Vol%]
R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
5
undisturbed soil samples (both for P and
M) start to loose water at higher pF-values
(pF 1,78) than the disturbed ones. Those
differences can be seen especially in the
amount of medium pores (MP).
60
Ph (1.2)
Ph (1.4)
Ph (1.6)
Ps
50
40
30
20
10
0
0
1
2
3
4
5
pF
60
Mh (1.2)
Mh (1.4)
Mh (1.6)
Ms
50
40
30
20
10
0
0
1
2
3
4
5
pF
Figure 1. Water retention curves as a
function of the bulk density for repacked
(Ph, Mh) and undisturbed (Ps, Ms) soil
samples
Figura 1. Curvas de retención de agua en
función de la densidad aparente para
muestras ensambladas (Ph, Mh) y no
disturbadas (Ps, Ms) de suelo
6
Physical properties in soils, Dec et al.
Table 2. Pore size distribution (wCP: >50 µm, nCP: 50-10 µm, MP: 10-0,2 µm, FP: >0,2
µm) and saturated hydraulic conductivity (Ks) as function of the bulk density (dB)
Cuadro 2. Distribución del tamaño de los poros (wCP: >50 µm, nCP: 50-10 µm, MP:
10-0,2 µm, FP: >0,2 µm) y conductividad hidráulica saturada (Ks) en función de la
densidad aparente
Treatment
dB
s
Pore size distribution [vol%]
KS
[Mg m ]
[vol%]
wCP
nCP
MP
FP
[cm s-1]
Ph(1.2)
1.18a
50.13c
15.07c
7.34d
9.68b
18.03a
-2.16 ± 0.05a
Ph(1.4)
1.39b
41.16b
8.37b
4.21c
7.33a
21.23b
-3.21 ± 0.07b
Ph(1.6)
1.62d
34.90a
0.51a
2.53b
6.26a
25.28c
-5.03 ± 0.29c
Ps
1.53c
n.d.
6.38b
1.26a
16.06c
18.12a
-3.18 ± 0.12b
Mh(1.2)
1.21a
47.65c
14.28c
7.64c
10.75ab
14.98a
-3.62 ± 0.08a
Mh(1.4)
1.42b
41.45b
8.08b
8.34c
7.63a
17.39ab
-4.41 ± 0.13b
Mh(1.6)
1.63c
34.60a
1.22a
4.61b
8.83a
19.93b
-5.12 ± 0.17c
Ms
1.47b
n.d.
5.64b
1.17a
20.77c
16.44a
-3.64 ± 0.32a
-3
In the same treatment, different letters indicates significant differences (Tukey test, p≤0.05).
Ph -homogenized soil samples prepared from Plough; Ps -structured soil samples taken
from Plough; Mh -homogenized soil samples prepared from Mulch; Ms-structured soil
samples taken from Mulch; n.d. -not determined; ± - standard error
The shrinkage curves show the relation
between water content and soil volume
changes (Fig. 2). Generally, the void ratio
(e) and the moisture ratio ( ) at saturation
decrease with increasing dB. Additionally,
with increasing dB the shrinkage curves
become shorter and smoother. The residual
shrinkage predominates in all investigated
samples, i.e. volume decrease is lower than
water losses, which can be explained by
high bulk density and the used water tension
(till -500 hPa).
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
1
1,2
0
1,0
Ph
0,8
dBi (1.2)
dBi (1.4)
dBi (1.6)
0,6
0,4
0,2
0,2
0,4
0,6
0,8
1,0
3
1,2
3
d0 [vol%]
1,4
Moisture ratio (cm /cm )
-3
dBi (1.2)
dBi (1.4)
dBi (1.6)
0,5
1,0
Ph
1,5
2,0
2,5
2,0
2,5
3,0
pF
1,4
1
1,2
0
1,0
Mh
0,8
dBi (1.2)
dBi (1.4)
dBi (1.6)
0,6
0,4
0,2
0,2
-2
-5
0,0
1,4
7
-1
-4
d0 [vol%]
3
3
Void ratio (cm3/cm
) ratio Void ratio (cm3/cm
) ratio
Void
Void
cm
(
cm
(
3
/cm
3
3
)
3
)
/cm
R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
-1
-2
-3
-4
0,4
0,6
0,8
1,0
3
1,2
1,4
3
Moisture ratio (cm /cm )
-5
0,0
dBi (1.2)
dBi (1.4)
dBi (1.6)
0,5
1,0
Mh
1,5
3,0
pF
Figure 2. Shrinkage curves of homogenized
repacked material prepared from Plough
(Ph) and Mulch (Mh). Bars indicate ± 1
standard error
Figure 3. Relative water content differences
as a function of the bulk density for repacked
soil samples Plough (Ph) and Mulch (Mh).
Bars indicate ± 1 standard error
Figura 2. Curvas de contracción del material
ensamblado preparado a partir de Plough
(Ph) y Mulch (Mh). Las barras indican ± 1
error estándar
Figura 3. Diferencias relativas en el
contenido de agua en función de la densidad
aparente para muestras ensambladas a partir
de Plough (Ph) y Mulch (Mh). Las barras
indican ± 1 error estándar
Although the soil does not present a great
shrinkage at the applied water tensions (Fig.
2) some structural changes due to water
menisci formation take place. These changes
can be evaluated through the relative water
content difference (dθ), understood as the
difference between the volumetric water
content with and without consideration of
the shrinkage in relation to the volumetric
water content at saturation (Fig. 3). The
dashed line represents the situation for the
soil assumed as a rigid body, when no
changes in the volumetric water content due
to shrinkage take place. As the bulk density
decreases, a greater deviation of dθ from
the dashed line is observed (it becomes more
negative) showing that structural changes
due to shrinkage become larger. By the
lower dB, the greater deviation of dθ occurs
near saturation (till -20 hPa), showing the
greater error in the determination of the
volumetric water content.
Effect of the bulk density on the hydraulic
conductivity
The saturated hydraulic conductivity
decreases with increasing bulk density as
a response of the smaller volume of coarse
pores in the disturbed samples (Table 2).
8
Physical properties in soils, Dec et al.
Ks for Ps and Ph (1.4) do not present
statistically significant differences, but Ps
has a greater bulk density, which normally
coincides with a smaller Ks value. The
same values for Ks can be explained by the
higher continuity between soil aggregates
of the undisturbed pore system at a given
bulk density. The undisturbed samples
collected from Mulch (Ms) present the same
d B and wCP as shown for Mh (1.4).
However, Ks of Ms is significant higher
than Mh (1.4), showing the pore-continuity
effect of the undisturbed material. In fact,
Ms is comparable in Ks with Mh (1.2) even
considering the higher d B of Ms.
As the soil drains, k/ψ curve decrease (Fig.
4). However, the intensity of these changes
differs depending on the bulk density and
pore-size distribution. Differences in k/ψ
observed in Ph at pF=3.0 reach almost one
order of magnitude depending on dB, while
for Mh this situation was not observed as a
result of the faster decrease of k/ at dB=1.4
than at 1.2 Mg m-3.
κ/ψ [log cm/sec]
-2
-4
-6
dB (1.2)
dB (1.4)
dB (1.6)
dB (1.2)
dB (1.4)
dB (1.6)
Ph
Mh
-8
-10
-12
-1-∞0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 -1 -∞0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
pF
pF
Figure 4. Simulated unsaturated hydraulic conductivity (k/ψ) for three bulk densities by
repacked soil samples for Plough (Ph) and Mulch (Mh)
Figura 4. Conductividad hidráulica no saturada simulada (k/ψ) para tres densidades
aparentes del suelo ensamblado a partir de Plough (Ph) y Mulch (Mh)
If the volumetric water content is corrected
by soil shrinkage, the estimated values of
unsaturated hydraulic conductivity increase
(Fig. 5). Thus it can be seen that if the soil
does not behave as a rigid body, errors in
the estimation of the volumetric water
content can be made, even in the residual
shrinkage range. These discrepancies
increase with decreasing bulk density (Fig.
3) and have consequences on the estimation
of the unsaturated hydraulic conductivity
(Fig. 5).
R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
κ/ψ [log cm/sec]
-2
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
dB (1.2a)
dB (1.2b)
-4
-2
-6
-8
-8
-10
-10
Ph
0
1
2
pF
3
dB (1.2a)
dB (1.2b)
-4
-6
4
9
Mh
0
1
2
pF
3
4
Figure 5. Effect of the soil shrinkage on the unsaturated hydraulic conductivity (a: modeled
k/ψ without consideration of shrinkage, b: k/ψ with consideration of shrinkage)
Figura 5. Efecto de la contracción del suelo sobre la conductividad hidráulica no saturada
(a: k/ψ sin considerar la contracción, b: k/ψ considerando la contracción)
DISCUSSION
Effect of soil structure on hydraulic
properties
Since the soil texture between undisturbed
and disturbed samples is identical,
differences in the relation between pore
volume and pore function are related to the
bulk density and soil structure formation.
The differences between pore size
distributions (or WRCs) of homogenized
(repacked) and undisturbed samples are the
consequence of soil aggregation. If repacked
and undisturbed soil samples collected in
Mulch are compared, differences in the WRC
at the same dB were assessed. Remarkable
is the higher amount of medium pores in
e.g. Ms than in Mh(1.4) which dominates
in the conservation tillage persisting over
longer periods. This differences tend to
decrease (but are in some cases still
significant) with increasing drainage,
showing the increasing effect of textural
pores. This latter effect was confirmed by
measurements made by Tuli et al. (2005).
Disturbed soil samples were only once
wetted and dried and, consequently no real
aggregation occurs. On the other hand, the
structure of the undisturbed soil is a
consequence of soil management and
successive swelling-shrinkage processes. As
Horn and Smucker (2005) suggested, the
aggregate formation and strength depend on
swelling and shrinking process and on
biological activity and kinds of organic
exudates as well as on the intensity, number
and time of swelling and drying events. The
greater differences observed between Mh
and Ms are probably related to the
conservation of soil structure avoiding the
aggregate destruction. This implies, that the
pore continuity under Mulch is not
interrupted allowing a more intensive water
transport. This can be confirmed by
investigations made by Buczko et al. (2006),
who observed higher hydraulically active
macropores and higher macropore
connectivity under conservation than
conventional tillage treatment. Lower
macropore connectivity in cultivated soils
were amongst others described by
Bodhinayake and Si (2004).
Tuli et al. (2005) investigated and compared
hydraulic properties of undisturbed and
10 Physical properties in soils, Dec et al.
disturbed (repacked) soil samples. With
respect to the WRC, they found out that
repacked soil samples present higher airentry pressure values than the undisturbed
ones, showing the lack of macropores in the
disturbed samples. In our results, the volume
of macropores (wCP) is the same for Ms
and Mh (1.4) but higher for Ps than Ph (1.6).
They measured also the air and water
permeability. For both investigated
properties, undisturbed soil samples showed
higher values. This illustrates the contribution
of more continuous macropores to water and
air permeability for the undisturbed samples.
In the same direction, Blackwell et al. (1986)
mentioned that differences in permeability
data between disturbed and undisturbed
samples are mostly governed by soil structure
and macroporosity. Generally, the amount
of macropores, are responsible of the
permeability of soil to water near saturation
(Iversen et al., 2001). However, the
continuity of these pores can not be
neglected. The greater Ks for the undisturbed
soil in M (Ms>Mh (1.4)) is influenced by
the pore continuity reached during the
aggregate formation, which does not take
place in the disturbed material.
An increasing bulk density in the
homogenized material implies a decrease of
coarse pores and an increase in middle and
fine pores. It is well known that this
behaviour induces changes on hydraulic
conductivity as assessed in structured soil
by Ankeny et al. (1990), Fuentes et al.
(2004), Horn and Smucker (2005). This
phenomenon appears as a result of the
reduction of structural pores (the large pores
are reduced at first, as mentioned by Richard
et al., 2001) and the destruction of pore
continuity developed during aggregate
formation. Saturated hydraulic conductivity
for soils with different amount of macropores
(and pore continuity) was also described by
Cameira et al. (2003), Fuentes et al. (2004)
and Buczko et al. (2006). They investigated
different soil types under two tillage
treatments and assessed lower Ks for
conventional than conservation treatment
responding to a lower soil aggregation (Six
et al., 1999; Pinheiro et al., 2004) and higher
disturbance of macropores continuity
(Osunbitana et al., 2005). Improvement of
pore continuity with increase of macropores
was also noticed by Currie (1965) who
investigated gas transport in highly developed
natural aggregated soils. He pointed out that
preferentially gas transport in soil profile
occurs through the inter-aggregate pore
system (more continuous than zones of
aggregated or crumb pores) formed by
macropores, being reduced in compacted
soils. The same trends were also described
by Fujikawa and Miyazaki (2005) for alluvial
soils.
Effect of bulk density on shrinkage
Regarding the importance of water for all
processes in soil, their hydraulic properties
are often investigated and well understood
if we assume a rigid pore system. Soils,
however, are no rigid bodies which results
in changes in their properties as a result of
internal and external forces. If we want to
understand the effect of shrinkage on
hydraulic properties, measurements on
disturbed and repacked soil samples can give
a good insight in the effect of non rigidity
on hydraulic functions. However, the
possibility of extrapolation to in situ
conditions can be rather restricted because
of the exclusion of water uptake effects
through roots on soil structure formation but
at least it represents the seedbed conditions.
Peng and Horn (2005) mentioned that soil
homogenization due to repeat ploughing
always results in an altered shrinkage pattern
in comparison with the identical soil without
any intermediate homogenization. The
ability of soil to shrink decreases exponential
with increasing d B , as far as the soil
particle/aggregates get closer. Horn et al.
(1991) showed that at a given dehydration
stresses, less structured soils were easy to
deform. If soils are compacted the amount
of contact points between particles and soil
strength increases (Hartge and Horn, 1999),
which results in a reduced mobility of
R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
particles to rearrange (Hartge, 2000).
Consequently, a more negative suction is
needed to regain structural changes. As the
soil drains the water menisci formation
induce shrinkage affecting the volume of
medium and fine pores (Peng and Horn,
2007) and the volumetric water content
(Figure 3). The later is underestimated and
consequently the simulation of k/ψ with the
van Genuchten is underrated as well. The
underestimation depends on the rigidity of
the pore system (determined by the cohesion
between particles and the angle of the
internal friction) and the applied water
suction (or intensity of drainage).
CONCLUSIONS
1. Structural changes affect the porous media
in their volume and function. These changes
get larger with decreasing bulk density.
2. Structural changes take place due to
internal (shrinkage) and external forces
(compaction) and affect the hydraulic
behaviour of these soils.
3. The aggregation of the soil is more intense
in undisturbed soil samples due to successive
wetting and drying cycles prefer the creation
of new pores.
4. The structural changes due to water loss
can be represented by dθ Without
consideration of dθ the values of k/ψ can be
underestimated.
REFERENCES
ANKENY, M. D., T. C. KASPAR, R.
HORTON. 1990. Characterization of
tillage and traffic effects on unconfined
infiltration measurements.
Soil.Sci.Soc.Am. J. 54:837-840.
BALL, B. C., D. J. CAMPBELL., J. T.
DOUGLAS., J. K. HENSHALL, AND
M. F. O’SULIVAN. 1997. Soil
structural quality, compaction and land
management. Eur. J. of Soil Sci. 48:
593-601.
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
11
BARTOLI, F., J. C. BEGIN, G. BURTIN
AND E. SCHOULLER. 2007.
Shrinkage of initially very wet soil
block, cores and cods from a range of
European Andosol horizons. Eur. J. of
soil Sci. 58: 378-392.
BLACKWELL, P. S., J. P. GRAHAM, J.
V. ARMSTRONG, M. A. WARD, K.
R. HOWSE, C. J. DAWSON AND A.
R. BUTLER. 1986. Compaction of a
silt loam soil by wheeled agricultural
vehicles. I. Effect upon soil conditions.
Soil Tillage Res. 7: 97-116.
BODHINAYAKE, W. AND B. C. SI. 2004.
Near -saturated surface soil hydraulic
properties under different land uses in
the St. Denis National Wildlife Area,
Saskatchewan, Canada. Hydrol.
Processes 18: 2835-2850.
BRONSWIJK, J. J. 1990. Shrinkage
geometry of a heavy clay soil at various
stresses. Soil. Sci. Soc. Am. J. 54:15001502.
BUCZKO, U., O. BENS AND R. F.
HÜTTL. 2006. Tillage effects on
hydraulic properties and macroporosity
in silty and sandy soils. Soil. Sci. Soc.
Am. J. 70:1998-2007.
CAMEIRA, M. R., R. M. FERNANDO
AND L. S. PEREIRA. 2003. Soil
macropore dynamics affected by tillage
and irrigation for a silty loam alluvial
soil in southern Portugal. Soil Tillage
Res. 70: 131-140.
CURRIE, J. A. 1965. Diffusion within soil
microstructure a structural parameter
for soils. Soil Sci. 16: 278-289.
DÖRNER, J. AND HORN, R. 2006.
Anisotropy of pore functions in
structured Stagnic Luvisols in the
Weichselien moraine region in N
Germany. J. Plant. Nutr. Soil Sci.
169:213-220.
12 Physical properties in soils, Dec et al.
FAO. 1998. World Reference Base for Soil
Resources. World Soil Resources
Report 84, Food and Agriculture
Organization of the United Nations,
Rome.
FUENTES, J. P., M. FLURRY AND D. F.
BEZDICEK 2004. Hydraulic properties
in a silt loam soil under natural prairie,
conventional till, and no-till. Soil. Sci.
Soc. Am. J. 68: 1679-1688.
FUJIKAWA, T., T. MIYAZAKI. 2005.
Effect of bulk density and soil type on
the gas diffusion coefficient in repacked
and undisturbed soils. Sol Sci. 170
(11):892-901.
Hartge, K. H. and R. Horn. 1989. Die
physikalische Untersuchung von
Böden. Stuttgart, Enke, 177 P.
HARTGE, K. H. AND R. HORN. 1999.
Einführung in die Bodenphysik. Enke,
Stuttgard, 304 S.
HARTGE, K. H. 2000. The effect of soil
deformation on physical soil properties.
A discourse on the common
background. P. 32-43. In R. Horn et al.
(ed) Subsoil compaction-distribution,
processes and consequences. Catena
Verlag. Reiskirchen, Germany.
HILLEL, D. 1998. Environmental Soil
Physics. Academic Press, London,
771 P.
HORN, R., BAUMGARTL, TH.,
KÜHNER, M. UND KAYSER, R.
1991. Zur Bedeutung des
Aggregierugsgrades für die
Spanungsverteiligung in strukturierten
Böden.
Zeitschrift
für
Pflanzenernährung und Bodenkunde
154: 21-26.
HORN, R., H. DOMZAL, A. S•OWI•SKAJURKIEWICZ,
C.
VAN
OUWERKERK. 1995. Soil compaction
processes and their effects on the
structure of arable soils and the
environment. Soil & Till. Res. 35: 2336.
HORN, R. AND A. SMUCKER. 2005.
Structure formation and its
consequences for gas and water
transport inunsaturated arable and forest
soils. Soil & Till. Res. 82: 5-14.
IVERSEN, B. V., P. MOLDRUP, P.
SCHJØNNING, AND P. LOLL. 2001.
Air and water permeability in differently
textured soils at two measurement
scales. Soil Sci. 166: 643-659.
KUTILEK, M AND D. R. NIELSEN. 1994.
Soil hydrology. Catena Verlag, 38162
Cremlingen-Destedt, Germany. ISBN
3-923381-26-3.
MCNABB, D. H., A. D. STARTSEV, AND
H. NGUYEN. 2001. Soil wetness and
traffic effect levels on bulk density and
air- field porosity of compacted boreal
forest soils. Soil. Sci. Soc. Am. J. 65:
1238-1247.
OSCHNER, T. E., R. HORTON, AND T.
REN. 2001. A new perspective on soil
themal properties. Soil Sci. Soc. Amer.
J. 65: 1641-1647.
OSUNBITANA, J. A., D.J. OYEDELEB
AND K.O. ADEKALUA. 2005. Tillage
effects on bulk density, hydraulic
conductivity and strength of a loamy
sand soil in southwestern Nigeria. Soil
and Till. Research 82: 57-64.
PENG, X. AND HORN R. 2005. Modelling
soil shrinkage curve across a wide range
of soil types. Soil Sci. Soc. Amer. J.
69: 584-592.
PENG, X., HORN R., S. PETH AND A.
SMUCKER. 2006. Quantification of
soil shrinkage in 2D by digital image
processing of soil surface. Soil and Till.
Research. 91:173-180.
PENG, X. AND HORN R. 2007.
Anisotropic shrinkage and swelling of
some organic and inorganic soils. Eur.
J. Soil. Sci. 58: 98-107.
R.C.Suelo Nutr. Veg. 8 (1) 2008 (1-13)
PINHEIRO, E. F. M., PEREIRA M. G.AND
L. H. C. ANJOS. 2004. Aggregate
distribution and soil organic matter
under different tillage systems for
vegetable crops in a Red Latosol from
Brazil. Soil and Tillage Research. I: 7984.
PLAGGE, R., RENGER, M. AND
CHRISTIAN H. ROTH. 1990. A new
laboratory method to quickly determine
the unsaturated hydraulic conductivity
of undisturbed soil cores within a wide
range of textures. Zeitschrift für
Pflanzenernähr. und Bodenk. 153: 3945.
RICHARD, G., COUSIN, I., SILLON, J.
F., BRUAND, A., GUERIF, J. 2001.
Effect of compaction on the porosity of
a silty soil: influence on unsaturated
hydraulic properties. Eur. J. Soil Sci.
52: 49-58.
J. Soil Sc. Plant Nutr. 8 (1) 2008 (1-13)
13
SIX. J., E.T. ELLIOTT AND K.
PAUSTIAN. 1999. Aggregate and soil
organic matter dynamics under
conventional and no-tillage systems.
Soil. Sci. Soc. Am. J. 63:1350-1358.
TULI. A., J. W. HOPMANS, D. E.
ROLSTON, AND PER MOLDRUP.
2005. Comparison of air and water
permeability between disturbed and
undisturbed soils. Soil Sci. Soc. Am. J.
69: 1361-1371.
VAN GENUCHTEN, M. TH. 1980. A
closed – form equation for predicting
the hydraulic conductivity of unsaturated
soils. Soil Sci. Soc. Amer. J. 44: 892898.